Question: Why do the shirts the contestants wear change from green to purple during the event?
Answer: The shirts don't change color, it's just the lighting that seems to give a different tint to them. The film switches from the typical movie lighting that gives the shirt a duller and darker look (which you may see as green), to the "live TV" lighting which brightens the shirt color to a blue (or purple) color.
Chosen answer: Jason Bateman talks about it in an interview. They didn't intentionally do it. When Bateman was casting kids for those parts, he picked based on talent or who he liked, not based on ethnicity. The parts and names were already written, but casting just didn't happen to match. Bateman also talked about how they were having trouble finding an Indian boy for the role of Chopra and wondered if the kid had to be Indian, although ultimately after reviewing Chand's audition tape, he decided to cast him.
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